(I) Prior Art Background
Many methods have been used in fracturing connecting rods, that include:    (i) Passing an electron beam along a desired splitting plane as in U.S. Pat. No. 3,751,080.    (ii) Providing holes in the fracturing plane through which the fracturing force is introduced as in U.S. Pat. No. 3,994,054    (iii) Using heat treatment or freezing to embrittle the fracture area as in U.S. Pat. No. 4,768,694    (iv) Applying a static or an impulsive force acting perpendicular to the fracture plane as in U.S. Pat. Nos. 4,860,419; 5,115,564; and 5,320,265.    (v) Actuating expanding mandrels using a wedge arrangement as in U.S. Pat. No. 5,503,317.
However, most of the known methods for fracturing the connecting rods are based on the same principle: application of an “outward pressure” to the crank bore till the generated stresses are high enough to fracture the connecting rod. Some of these methods attempted to overcome the difficulty of fracturing such high strength material by reducing or weakening the cracking area, by using techniques, such as, the cryogenic cooling and the electron beam hardening, which have a deleterious effect on material performance.
Since connecting rods are made of high strength materials, the fracturing force is required to be of big magnitude. The use of big force has a negative effect on the quality of the fractured connecting rod, especially, with large size connecting rods in a high production environment. Despite the improvements, some disadvantages still exist such as: plastic deformation, lack of flexibility in adapting the same technique to different sizes of connecting rods, repeated breakage of force exertion elements of the machine, and poor quality of the fractured connecting rod. Moreover, some techniques are slow, costly, and technically very elaborate.
Before presenting the idea of the current invention, it is necessary to discuss the engineering principles on which the invention stands.
(II) Technical Background
(A) Fracture Mechanics:
Strength failures of load bearing elements can be either of the yielding-dominant (ductile) or fracture-dominant (brittle) types. In case of a cracked element, it may fail due to reaching the plastic collapse or fracture condition. Collapse and fracture are competing conditions, and the one satisfied first will prevail.
High-strength materials are more likely to fail in fracture mode before attaining the plastic collapse strength. Since connecting rods are made of high-strength materials, they generally fail under tensile forces due to reaching the fracture limit state.
Fracture may take place under one of two conditions, namely, plane stress or plane strain, depending on the thickness of the element. In general, connecting rods are thick enough to sustain plane strain fracture. In the presence of a V-notch or a crack, fracture occurs under essentially elastic conditions with a limited plasticity zone at the tip of the crack.
The stress intensity factor (K), is the characterizing parameter for crack extension. For each stress pattern, there is a corresponding value of the stress intensity factor. When the stress intensity factor reaches a certain value, crack propagates and collapse by fracture occurs. That critical value of the stress intensity factor under plane strain conditions, called the Plane Strain Fracture Toughness (KIc), can be considered as a material property characterizing the crack resistance. Thus, the same value of KIc should be obtained for a given material while testing specimens of different geometric shapes and sizes.
Lower temperature and faster strain rate decrease the plane strain fracture toughness for a specific material, while increasing the length of a pre-existing crack or decreasing the fracturing area will increase the stress intensity factor, if all other factors remain unchanged.
(B) Resonance of a Structural System:
The connecting rod, with all movement and rotation constraints imposed on it during the fracturing process, can be viewed as a structural system. Before explaining how to achieve and make use of a resonance condition in this fracturing technique, it is helpful to introduce the following definitions pertaining to an idealized structural system with finite number of degrees of freedom:
Degrees of freedom: the number of independent displacements required to define the displaced positions of all the masses relative to their original positions is called the number of degrees of freedom (DOFs).
Natural mode of vibration: a multi-degree-of freedom system (MDOF) would undergo simple harmonic motion, without a change of the deflected shape, if free vibration is initiated by appropriate distribution of displacements in various DOFs. In other words, for some characteristic deflected shapes, the system would vibrate in simple harmonic motion, and the initial shape would be maintained through out the motion. Each characteristic deflected shape (Φn) is called a natural mode of vibration of the MDOF system.
Natural vibration properties: the time (Tn) required for a system to complete one cycle of the simple harmonic motion in one of its natural modes is called the natural period of that particular vibration mode. The corresponding natural cyclic frequency of vibration is fn, and the natural circular frequency of vibration is ωn, where:Tn=2π/ωn=1/fn.A vibrating system with N number of DOFs has N natural vibration frequencies ωn (n=1, 2, . . . , N), arranged in sequence from smallest to largest (ω1<ω2< . . . <ωN), with corresponding natural periods Tn, and natural modes Φn.
The excitation frequency: the frequency of a harmonic force applied to a system is called the excitation frequency or the forcing frequency.
Damping: the process by which vibration steadily diminishes in amplitude is called damping.